As shown in FIG. 1, two key components exist in a packet-switched network: Media Gateway Controller (MGC) and Media Gateway (MG). The MGC is responsible for the call control function, and the MG is responsible for the service bearer function. In this way, the call control plane is separated from the service bearer plane, the network resources are sufficiently shared, equipment upgrade and service extension are simplified, and the development and maintenance costs are reduced.
The (media) gateway control protocol is a protocol mainly used for communication between the MG and the MGC. The currently prevalent (media) gateway control protocols include H.284/Gateway Control Protocol (MeGaCo) and Media Gateway Control Protocol (MGCP). The MGCP R1 protocol was formulated by the Internet Engineering Task Force (IETF) in October 1999 and revised in January 2003. The H.248/MeGaCo R1 protocol was formulated by the IETF and the International Telecommunications Union (ITU) in November 2000 and revised in June 2003. The H.248 R2 protocol was formulated by the ITU in May 2002 and revised in March 2004. The H.248 R3 protocol was formulated by ITU in September 2005.
Taking the H.248 protocol as an example, various resources on the MG are abstractly represented by terminals. Terminals are categorized into physical terminals and ephemeral terminals. Physical terminals are physical entities which exist semi-permanently, for example, Time Division Multiplex (TDM) channels. Ephemeral terminals represent the public resources which are requested temporarily and released soon after being used, for example, Real-time Transport Protocol (RTP) streams. Moreover, a root terminal is used to represent the entirety of the MG. The combinations among terminals are abstractly represented by contexts. A context may include multiple terminals. Therefore, topology is used to describe the interrelations between terminals. The terminal not related to other terminals is contained by a special context called “null”.
Based on an abstract model, such as the foregoing protocol, the call routing is actually an operation on the terminal and the context. Such operations are performed through Command Requests and Reply between the MGC and the MG Command types include: Add, Modify, Subtract, Move, AuditValue, AuditCapabilities, Notify, and ServiceChange. Command parameters, also known as descriptors, are categorized into Property, Signal, Event, and Statistic. The parameters with service relevance are logically aggregated into a package.
Taking the IP multicast technology of sending services to type-D IP address (namely, multicast address) as an example, when the IP multicast technology enables the sender to send the same service content to one or more receivers, only one copy of the same content needs to be sent to the specified multicast address, thus reducing the load on the service sender and increasing the transport network efficiently.
In order to obtain the multicast contents, the content receiver (namely, the user) joins a service multicast group, for example, by using the Internet Group Message Protocol (IGMP), to require the adjacent router to send service contents to the content receiver. Routers interact with each other through a multicast routing protocol, for example, Protocol Independent Multicast-Sparse Mode (PIM-SM) protocol, to create a multicast forwarding path. In this way, the multicast service contents can be sent to the content receiver from the multicast source along the multicast forwarding path.
When a service stream is transferred through the multicast technology, the service sender needs to send only one service stream regardless of the number of receivers. Only one data stream is generated along the transmission path from sending point of the multicast data to the transfer point of the receiver. Accordingly, the multicast technology reduces the load of the service sender (service provider), and makes better use of network resources.
In the propagation path of the current multicast content, the entity for receiving and forwarding the multicast service is a router. When different networks are interconnected, an MG is usually required. However, the action of an MG is not identical to the action of a router. First, the MG is unable to receive multicast routing messages such as PIM-SM, and hence the multicast data is unable to pass through the MG. The H.248 protocol is increasingly becoming a mainstream protocol for bearer control, and needs to support multicast media and signaling forwarding.